The present invention generally relates to printing technology, and more specifically to the production and use of ink compositions having chemical additives therein which substantially improve print quality.
Significant developments have been made in the field of electronic printing technology. Specifically, a wide variety of highly efficient printing systems currently exist which are capable of dispensing ink in a rapid and accurate manner. Thermal inkjet systems are especially useful for this purpose. Thermal inkjet printing cartridges basically include an ink reservoir in fluid communication with a substrate having a plurality of resistors thereon. Selective activation of the resistors causes thermal excitation of the ink and expulsion thereof from the ink cartridge. Representative thermal inkjet systems are discussed in U.S. Pat. No. 4,500,895 to Buck et al.; U.S. Pat. No. 4,513,298 to Scheu; U.S. Pat. No. 4,794,409 to Cowger, et al.; the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985); and the Hewlett-Packard Journal, Vol. 39, No. 4 (August 1988), all of which are incorporated herein reference.
Notwithstanding the existence of advanced printing systems, additional research has also been conducted in order to produce new and effective ink products to be used in these systems. Unless ink compositions are properly formulated, problems can occur with respect to ink delivery and image quality. These problems are especially acute in cartridge systems which do not include a foam or sponge-type reservoir system for holding ink therein. For example, it is important for ink products to be specially formulated so that the surface tension levels thereof are not excessively high. Ink compositions having excessively high surface tension levels (e.g. +60 dynes/cm) often produce graphics having a low degree of quality and resolution. In addition, applied ink compositions having high surface tension values may mottle or coalesce on the paper or other print media being used, thereby causing blotchy images. Ink compositions having high surface tension values frequently promote bubble adhesion/longevity within the firing chambers of thermal inkjet cartridges and other printing systems. Bubble adhesion/longevity involves the formation of bubbles in the ink which do not dissipate or dissipate at a slow rate. This problem again results in impaired cartridge operation.
In an attempt to control the above problems, surfactants have been added to the ink compositions. Exemplary surfactant materials suitable for this purpose include polyethylene glycol; N,N-Dimethyldodecyl amine-N-oxide; 3-(N,N-Dimethylpalmityl-ammonio)-propanesulfonate, and a commercially available product from the Air Products Company of Philadelphia, Pa. sold under the trademark Surfynol 465 which has the following structure: ##STR1##
While surfactants decrease the surface tension of ink compositions, they do not necessarily control other problems which may occur during application of the compositions. For example, lowering the ink surface tension generally has no predictable effect on steady state ink drop mass per firing. The phrase "ink drop mass per firing" as used herein basically involves the amount of ink ejected from a nozzle in an ink cartridge during one firing thereof, and is a function of many variables. These variables include but are not limited to the geometry of the firing chamber in the cartridge, the resistor design used in the cartridge, the cartridge input energy, the designated firing frequency, and the physio-chemical properties of the ink materials (e.g. ink wettability).
Decreasing the surface tension of ink compositions does not necessarily effect drop mass per firing because the other factors indicated above may play a more significant role. Ink compositions with a low drop mass per firing are delivered to print media in an insufficient quantity, resulting in low contrast images. This occurs because the amounts of chemical dyes and other important materials in these compositions are insufficient to cover the desired areas of the print media to produce clear, dark images.
Accordingly, a need remains for an ink composition which is specifically formulated to have a low surface tension level while maintaining a high level of ink drop mass per firing. The ink composition should also be suitable for use in a wide variety of printing systems, including thermal inkjet units. The present invention satisfies these needs in a unique and effective manner, as described herein below.